The given PHP example helps you to add the thumbnails to RSS feed into WordPress. Code Example:
<?php
function diw_post_thumbnail_feeds($content) {
global $post;
if(has_post_thumbnail($post->ID)) {
$content = '<div>' . get_the_post_thumbnail($post->ID) . '</div>' . $content;
}
return $content;
}
add_filter('the_excerpt_rss', 'diw_post_thumbnail_feeds');
add_filter('the_content_feed', 'diw_post_thumbnail_feeds');
?>
RSS Feed